Radio frequency identification tags and radio frequency identification tag systems are known, and find numerous uses. For example, radio frequency identification tags are frequently used for personal identification in automated gate sentry applications protecting secured buildings or areas. Information stored on the radio frequency identification tag identifies the person seeking access to the secured building. A radio frequency identification tag system conveniently provides for reading the information from the radio frequency identification tag at a small distance using radio frequency (RF) data transmission technology. Most typically, the user simply holds or places the radio frequency identification tag near a base station that transmits an excitation signal to the radio frequency identification tag powering circuitry contained on the radio frequency identification tag. The circuitry, responsive to the excitation signal, communicates the stored information from the radio frequency identification tag to the base station, which receives and decodes the information.
In general, radio frequency identification tags are capable of retaining and, in operation, transmitting a substantial amount of information--sufficient information to uniquely identify individuals, packages, inventory and the like. The radio frequency identification tag is also capable of receiving and storing information. In a read/write application, the base station is not only capable of sending an excitation signal and receiving a response from the radio frequency identification tag, but it is also capable of sending a data, or write, signal to the radio frequency identification tag. The radio frequency identification tag receives the write signal, which may contain data to be stored within the tag, a code or a command. Depending on the type of write signal, the radio frequency identification tag responds accordingly, such as by storing the data or acting upon the command.
To couple either the inductive or electrostatic signals between the base station and the radio frequency identification tag, the tag necessarily includes an antenna having at least one and frequently two antenna elements. Typically, a tag circuit chip and the antenna are electrically coupled and bonded to a tag substrate. The tag may also include additional components, for example, resistors, capacitors, inductors, etc. that must also be electrically coupled to the tag circuit chip and/or the antenna. Conventional tag design provides conductive traces formed on a substrate with the tag circuit chip, components and antenna bonded to the substrate and electrically coupled to the conductive traces. Wire bonding is a common technique for providing an electrical couple between the interconnection pads on the tag circuit chip and/or the component and the conductive traces. Alternatively, "flip" chip technology provides raised conductive regions ("bumped pads") on the tag circuit chip (and similarly on the electrical components). The "flip" chip, during assembly, is inverted and positioned to the substrate with the bumped pads aligning with and electrically coupling to the conductive traces. A conductive adhesive may be used between the bumped pads and the conductive traces to ensure a good electrical couple as well as to supplement the mechanical adhesion of the tag circuit chip to the substrate.
As will be readily appreciated, larger interconnection pads on the tag circuit chip provide more area for coupling between the tag circuit chip and the conductive traces. Additionally, larger interconnection pads on the tag circuit chip makes aligning the tag circuit chip with the conductive traces easier. However, large interconnection pads are expensive. For example, using photomasks, plating (whether electrode or electrode-less) and similar metalization techniques to form larger interconnection pads can range in cost from $50 to $150 per chip wafer. This equates to a cost of about 2-5 cents per tag circuit chip when separated from the wafer.
In a number of radio frequency identification tag applications, the radio frequency identification tag is designed as a single use, disposable device. For example, in electronic article surveillance applications, a radio frequency identification tag is attached to and remains with each item in inventory being tracked. Hundreds of millions of items are tracked using electronic article surveillance technology in shops, stores and warehouses around the world. It is also proposed to use radio frequency identification tag technology in mail and package delivery tracking applications. The United States Postal Service alone handles over 600 million pieces of mail each day. At even the lowest estimate of approximately 2 cents per radio frequency identification tag circuit chip, increasing the size of the interconnection pads using known plating techniques is excessively cost prohibitive.
Thus there is a need for an improved radio frequency identification tag circuit chip.